Description: Define a function on topologies whose value is the convergence relation
for sequences into the given topological space. Although f is
typically a sequence (a function from an upperset of integers) with
values in the topological space, it need not be. Note, however, that
the limit property concerns only values at integers, so that the
real-valued function ( x e. RR |-> ( sin( _pi x. x ) ) )
converges to zero (in the standard topology on the reals) with this
definition. (Contributed by NM, 7-Sep-2006)